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NOV- 20, 1962
'r. M. DAUPHINEE
3,065,418
VERNIER POTENTIOMETER
Filed Jan. 4, 1960
3 Sheets-Sheet l
|NVENTOR
THOMAS MDAUPHINEE
BYMWEW
ATTORNEYS.
Nov. 20, 1962
3,065,418
T.M.DAUHHNEE
VERNIER POTENTIOMETER
Filed Jan. 4, 1960
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INVENTOR
THOMAS MDAUPHINEE
BY M ‘113%
ATTORNEYS.
Nov. 20, 1962
T. M. DAUPHINEE
3,065,418
VERNIER POTENTIOMETER
Filed Jan. 4, 1960
3 Sheets-Sheet 5
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INVENTOR
THOMAS MDAUPHINEE
BY
ATTORNEYS.
.ice
and
3,055,4l8
Patented Nov. 20, 1962
2
?rst series circuit including a source of direct current and
3,665,415
a rheostat for varying this current, ?rst and second tapped
Thomas M. Dauphinee, @ttawa, @ntario, dlanada, assign
resistance
second series
elements
circuit and
is composed
a ?xed resistance
of a fixedelement.
proportion
or to National Research ifouncil, Ottawa, Qntario,
Qanada
Filed Ean. 4, 19nd, Ser. No. 242
6 (Ilaims. (Qi. 324-93)
therealong, a ?rst current limiting resistor and a variable
PECSTENTEGM "5BR
of the ?rst tapped resistance element variably situated
portion of a third tapped resistance element. Similarly,
a third series circuit is composed of a ?xed proportion of
This invention relates to a Vernier potentiometer ca
said second tapped resistance element variably situated
pable of measuring unknown voltages to six signi?cant 10 therealong, a second circuit limiting resistor and a vari
?gures.
able portion of a fourth tapped resistance element. A
?rst galvanometer and a pair of terminals adapted to be
Vernier Potentiometers are well known in the prior art
but the construction of a potentiometer readable to six
connected to a second source of direct current are con
?gures and capable of measuring low voltages, say to 1A9
nected in series across the combination of the third and
of a micro-volt, presents major problems if the readings 15 fourth tapped resistance elements. A standard cell and
are to be reproducible and accurately known.
A major source of error is introduced by the changes
of resistances of current-carrying switches which are lo
cated between the points or" connection of the potential
?xed resistance. The standard cell and the first source
of direct current have the same polarity relative to the
?xed resistance. in addition, a switching means compris
terminals or which otherwise affect the current distribu~
tion in the potentiometer network. This source of error
ing a plurality of pairs of terminals is provided. One
pair of the terminals is adapted to be connected to the
can only be eliminated by making switch resistance varia
tion negligibly small. The permissible variation in re
sistance at the switch contacts depends upon the particu
lar circuit involved, but there has been great dif?culty in
developing circuits with reasonable switch requirements
which are not unstable for other reasons.
Another source of error in measuring very small volt
ages occurs as a result of deviations from the nominal
value of the resistors used in measuring networks. These 30
changes may be caused by temperature variations, by
permanent changes in resistance resulting from aging or
a second galvanometer are connected in series across the
second source of direct current and the ?rst current sens
ing device which
Elli?
corrected in series. A plurality of
reference resistance elements and the four mentioned
tapped resistance elements are connected in different com
binations across the remainder of the terminal pairs. One
of the combinations includes a speci?ed one of the tapped
resis‘ance elements wherein a speci?ed group of taps is
to be tested.
One advantage obtained by my invention is that the
switch resistance is less critical than in potentiometer cir
cuits known in the prior art. This occurs as a result of
the fact that the current carried through each of the
switch contacts is reduced by a large factor. This means
that the switch resistances can be much greater, and still
strains in the wire, or by inaccuracies in the original ad~
justment of the resistors. if all of the coils of an instru
ment change identically, no error results, but it is quite
unreasonable to expect that this will occur to six signi?
not cause a critically large voltage drop across the con
cant ?gures. As a result, some form of calibration Will
tacts. in a practical application of my circuits, the switch
usually be necessary, and should be available, if only for
stability requirements have been reduced. from 2x10-5
checking purposes. Since it is unlikely that another
ohms for the usual six ?gure instrument to 5X‘10—3
instrument of su?cient precision will be available to ef 40 ohms with fewer switch contacts in direct series with the
current'sensing element. The switch tolerances of this
feet the calibration, autocalibration, that is, intercornpari
son of coils of the instrument itself, is preferable. With
circuit are such that a six ?gure instrument can have a
decade type instruments, autocalibration is relatively ac
safety factor in the switches of at least 25 over a com
curate, since each of the 10 resistors of the decade is
parable ?ve ?gure instrument of conventional design and
being compared with the whole of the next lower decade.
special switch considerations are therefore completely un
necessary.
However, if a 160‘ step dial is used in place of the decade
arrangement, a special problem is presented, since the ac
Another advantage of my invention is that it has proved
cumulated error over 169* steps becomes undesirably
quite easy to make provisions for autocalibration of dials
large. Autocalibration in, say, 10 groups of 10 steps each
in subgroups of four or ?ve resistors for the ?rst resist
is desirable, but no simple system has appeared to date.
ance element and it} for the second and third resistance
Thermal electrornotive forces in switches and leads, or
elements, thus providing a number of “?xing" points along
the resistance elements which greatly reduce the prob
across the resistors, introduce a third source of error.
Thermal electromotive forces in switches are likely to be
ability of accumulative errors in the calibration.
relatively large at the moment of operation, due to fric
Another advantage or" my invention is that only two
tion heating, but usually die away exponentially to a
of the switch contacts are in series with the current sens
small, but not necessarily negligible, value quite soon
ing element and the second source of voltage. Since
thereafter. Thermal eleetromotive forces in the Wiring
these are the ?rst operated switches, any thermal electro
of the instrument may be prevented by ensuring uni
motive forces developed during their operation should
formity of temperature throughout, by using materials
with low thermoelectric power relative to one another
have died out by the time a balance of the last two re
sistance elements is achieved. All other contacts are so
and by making joints between dissimilar metals in pairs
located in the circuit that thermal electromotive forces
in such a way that residual thermal electromotive forces
tend to cancel out. However, in the prior art Vernier
are attenuated to negligible values.
potentiometers, the operating switches are in series with
My invention will be further described with reference
to the accompanying drawings in which:
the measuring circuit so that their thermal electromotive
FIGURE 1 is a schematic diagram‘ of a Vernier poten
forces, including those of the last operated switch, are
tiometer known in the prior art;
FEGURE 2 is a schematic diagram of a Vernier poten
tiometer according to my invention; and
directly introduced into the measurement. in a partially
successful attempt to overcome this fault, some poten
tiometers may be oil ?lled to reduce heating in operation
FIGURE 3 is a schematic diagram of a Vernier poten
70
am. to carry the heat away quickly.
tiometer having autocalibration means according to my
In my invention, a Vernier potentiometer comprises a
invention.
’
'
3,065,418
4
FIGURE 1 is a schematic circuit diagram of a six ?g
ure vernier potentiometer known in the prior art in which
the source of direct current 1 is connected in series with
rent ?owing from the direct current source 1. It will be
appreciated that the accuracy of this determination is
dependent upon the stability of the resistance in the
galvanometer circuit which includes the resistance at con
a current controlling rheostat 2. Through conductors 3
and 4, this combination is connected in series with a
?rst tapped resistance element '5 and thence through con
proportion of the current ?owing from the direct cur
ductor 6 to a ?xed resistance element 7 which completes
rent source 1 the source or" error produced becomes much
tacts 12 and 13 and that since these contacts carry a large
the circuit to the negative terminal of the direct current
more important as the accuracy or" the measurement of
the unknown source of voltage becomes greater.
source 1. A balancing galvanometer 8 and a standard
FTGURE 2 is a schematic diagram of a six ?gure
cell 9 are connected in series across the terminals of the 10
?xed resistance element 7. The ?rst tapped resistance
element 5 is made up of a series of individual equal
Vernier potentiometer according to my invention, in which
a ?rst source or" direct current 31 is connected in series
with a rheostat 32, a conductor 33, a ?rst tapped resist
ance element 34, a second tapped resistance element 35,
contacts 12 and 13 are adapted to be moved. Contacts
12 and 13 are in ?xed spaced relation to each other, the 15 and a ?xed resistance element 36 to complete the circuit
to the negative terminal of the direct current source 31.
spacing being equivalent to two of the resistors of the
The terminals of a galvanometer 37 and of a standard
?rst tapped resistance element 5. A second tapped resist
cell 35 are connected in series across the ?xed resistance
ance element 14 having a resistance equal to the sum of
resistors 11 having resistance taps 111 along which sliding
36 such that the polarities of the standard cell 38 and the
two individual resistors 11 of ?rst tapped resistance ele
ment 5 is connected between the contacts 12 and 13. The 20 ?rst direct current source 31 are the same relative to
the ?xed resistance 36. The ?rst tapped resistance ele
second tapped resistance element 14 is comprised of a
ment 34 is composed of a series of equal resistances 39
series of resistors 16 having taps 15 along which a con
having equally spaced taps 411 along which contacts 41
tact 17 is adapted to move. Connected in parallel with
and 42 are adapted to slide. Contacts 41 and 42 are in
the ?rst tapped resistance element 5 is a third tapped
resistance element 213. The parallel circuit is composed 25 ?xed spatial relation with each other, the spacing being
of a conductor 18, a current limiting resistor 19, a third
tapped resistance element 29 and a conductor 21. The
third tapped resistance element 20 is made up of a series
of resistors 22 having individual taps 23 along which a
contact 24 is adapted to move. Between moving con
tacts 17 and 24, a balancing galvanometer 25 and a pair
of terminals 26 and 27 are connected in series.
A sec
ond source of direct current, whose value is to be deter
mined, is connected across the terminals 26 and 27.
a distance equal to that between two taps 4t) separated by
the two connected resistances 39 or" the ?rst tapped resist
ance element 34.
A current limiting resistor 43 is con
nected in series with the contact 41. A third tapped
resistance element 44 is made up of two identical resistors
4% and 45, respectively. A sliding tap 47 is in contin
ous engagement with both of the identical resistors 43
and 45 so that the current through the combination re
mains constant and independent of the position of con
In operation, current flowing from the direct current 35 tact 47. The resistor 48 is connected in series with cur
rent limiting resistor 43 which in turn is connected to
source 1 is controlled by the rheostat 2 and flows through
the contact 41. The other resistor 45 is connected di
conductor 3, then dividing between conductors 4 and 18.
rectly in series with the other contact 42 and is made up
Because of the presence of the current limiting resistor 19
of the series of individual identical resistors 45a having
in conductor 18, the amount of current carried by this
circuit is much less than that carried through conductor 40 taps 46 along which the contact 47 is adapted to slide.
The second tapped resistance element 35 is composed
4 and the ?rst and second tapped resistance elements 5
of a series of identical resistors 49 having individual
and 14, respectively. The current in conductor 4 di
equally spaced taps 5% along which sliding contacts 51
vides equally between the particular resistor 11 and the
second tapped resistance element 14 which are connected
and 52 are adapted to move.
in parallel. Since the current ?owing in this part of
the circuit is relatively large, the resistance at the con
?xed space relation to each other, the spacing being
equal to that between two adjacent taps 51) of the second
tapped resistance element 35. A current limiting resistor
tacts 12 and 13 must be extremely small to prevent a rela
Contacts 51 and 52 are in
tively large voltage drop from appearing across these con
tacts. Using the ?rst tapped resistance element 5, the
53 is connected in series with one of the contacts 52. A
fourth tapped resistance element 54 is connected in se
contacts 12 and 13 are moved from left to right to in 50 ries with the contact 51, the contact 52, and the current
crease the voltage appearing at contact 17. Since the
limiting resistor 53. The fourth tapped resistance ele—
sum of the resistances 16 in the second tapped resistance
ment 54 is made up of a series of identical resistors 55
having taps 56 along which a contact 57 is adapted to
element 14 is equal to the sum of the resistances 11 of
the ?rst tapped resistance element 5 which are bridged by
the sliding contacts 12 and 13, the sliding contact 17
moving along the second resistance element taps 15 may
vary the voltage by an amount no greater than that ap
pearing between the contacts 12 and 13.v
The remainder of the current from conductor 3 ?ows
through the conductor 18 and the current limiting resistor
19 to the third tapped resistance element 20 where a volt
age is selected depending upon the position of a sliding
contact 24 along a series of taps 23, the voltage selected
being larger as the contact 24 is displaced from left to
right. Inititally, the standard cell 9 is introduced in the
circuit and a balance is obtained using the galvanometer
8. Maintaining the constant current from the ?rst source
of direct current 1, a second source of direct‘ current
move.
One end 53a of the tapped resistance 45 is connected
through a conductor 58 to a balancing galvanometer 60
which is in series with terminals 61 and 62 which are
adapted to be connected to a second source of direct
current whose value is to be determined. These ter
rninals in turn are connected through a conductor 59 to
one end 59a of the fourth tapped resistance element 54.
In operation, current from the direct current source 31
is controlled by the rheostat 32. The standard cell 38
is introduced into the circuit and rheostat 32 adjusted
until the net current ?owing through the galvanometer
37 is zero. The current through galvanometer 37 will be
zero when the voltage developed across ?xed resistance
by current from the direct current source 31 is equal to
the voltage of the standard cell 38. Then the second
whose value is to be determined is introduced between
the terminals 26 and 27 and the ?rst, second, and third 70 source of direct current whose value is to be determined
resistance elements 5, 14 and 211 respectively are adjusted
is introduced across the terminals 61 and 62 with a
in that order until galvanometer 25 indicates that bal
polarity as indicated and the ?rst, second, third and
ance has been achieved, whereupon the value of the un
fourth tapped resistance elements 34, 35, 44 and 54,
known voltage may be determined from the settings of
respectively, are adjusted until the net current ?owing
the tapped resistance elements and the value of the cur
through the galvanometer 6%) is zero. The current
5
3,065,419
6
through galvanometer 66 mil be zero when the voltage
developed across the conductors 58 and 59 is equal, but
of opposite polarity to that of the second source of di
resistance 70 was made equal to four of the individual
resistances 39 of the ?rst tapped resistance element 34
and the second tapped resistance element 35 was made
rect current introduced across the terminals 61 and 62.
equal to one resistance 39 of the ?rst tapped resistance
The voltage developed across conductors 58 and 59 is
determined by the current from direct current source 31
element 34, so that when contacts 51 and 52 are in their
flowing through the combination of the four tapped
resistance elements 34, 35, 44 and 54. The value of
the second source of direct current can be determined
extreme left hand position as shown in FIGURE 3, ?ve
taps of the ?rst resistance element 34 are compared to
the sum of the second tapped resistance element 35 and
the part 70 of the ?xed resistance element 36. How
from the settings of the tapped resistance element and 10 ever, if the contacts 51 and 52 are moved to their ex
treme right position then the part 76 of the ?xed resist
from the known value of current from the direct cur
rent source 31.
ance 36 alone acts as the reference resistor and will be
According to my invention, the ?rst and second tapped
balanced by approximately four taps of the ?rst ?xed
resistance element 34. By calibrating in groups of four
resistance elements 34 and 35 are connected directly
in series with the ?rst source of direct current 31 and 15 or ?ve, a series of calibration points are obtained along
carry by far the larger portion of that current Without
the need of sliding contacts and the relatively large volt
the resistance chain and it is then possible to calibrate
the individual resistors 39 located between these points
age drop which would occur across such contacts due
against the whole of the second ?xed resistance element
to the large current. The larger the voltage of the sec~
35 acting as the reference resistor.
ond direct current source introduced across the terminals 20
The calibration of second tapped resistance element 35
61 and 62 the farther the taps 41, 42 must be displaced
is conducted in a similar manner by ?rst connecting slid
to the left or the farther the taps 51, 52 must be displaced
ing contacts 64 and 65 across terminals 71 and 72. In
to the right to balance the galvanometer 60. Because
this position a reference resistor 74 which forms part of
of the presence of current limiting resistors 43 and 53 in
the ?rst tapped resistance element 34 is introduced into
series with the third and fourth tapped resistance elements 25 the circuit and balance is obtained on the galvanometer
44- and 54, the current carried by these elements is rela
60 by varying the current from the direct current source
tively small and thus, the voltage drops across the con
31, using rheostat 32. The contacts 64 and 65 are then
tacts 41, 42 and 51, 52 as well as 47 and 57, are so
switched back to “normal” terminals 66 and 67, and with
small that the determination of the unknown voltage to
tapped resistance elements 34, 44, and 54 remaining at
six signi?cant ?gures is possible. After the ?rst and sec 30 zero, the contacts 51 and 52 are moved along the sec
ond tapped resistance elements 34 and 35 have been ad
ond tapped resistance element 35 until balance is achieved.
justed, the third tapped resistance element 44 is adjusted
Final balance is carried out as before. In a properly
to introduce increasing voltage by moving the contact 47
constructed instrument the difference will never be more
from right to left. In the case of the third tapped resist
than a few steps in the last dial. In the potentiometer
ance element 44, the resistor 48 is used to maintain the 35 as constructed, where the second tapped resistance ele
current constant and so avoid the slight non-linearity of
ment 35 was composed of 100 identical resistance steps
voltage, and change of overall potentiometer current, both
formed by resistors 49, the reference resistor 74 was made
of the order of a few parts per million of full scale read
equivalent to 10 of the resistors 49, and in this way, 10
ing, that would otherwise result as the sliding contact 47
calibration points were obtained along the tapped resist
is moved. This re?nement is not required for the fourth
ance 35. Each of the individual resistors 49 between
tapped resistance element 54, which is last adjusted by
the established reference points were then individually
moving it from left to right to provide increasing voltage.
compared against the whole of the third tapped resistance
FEGURE 3 is a schematic diagram of a Vernier poten
element 44 which acted as a reference resistor equivalent
tiometer according to my invention incorporating an auto
to a single resistance 49 in the second tapped resistance
calibration means. The same potential measuring circuit
element 35.
is used in this ?gure as in FIGURE 2, and the same nu~
A similar method is used to calibrate the third tapped re
merical designations have been used for like parts in both
sistance element 44. Sliding contacts 64 and 65 of switch
?gures. A multi-contact switch 63 has been introduced
63 are placed across terminals 75 and 76 which introduces
into the circuit and the balancing galvanometer 6t} and
a reference resistor 78 into the balancing circuit. In the
the terminal pair 61, 62 are connected in series across 50 potentiometer as constructed, the reference resistor 78
the sliding contacts 64 and 65 of the switch 63. In
was equivalent to 10 of the individual resistances 45a
normal potential measuring operations, the contacts 64
which forms part of the 100 tapped resistance 45. As
discussed previously, the 10 calibration points were ob
tained along the tapped resistance element 44 and then the
For the calibration of the ?rst tapped resistance ele 55 individual resistors 45a located between these points were
ment 34, the other tapped resistance elements 35, 44, and
compared with the whole of the fourth tapped resistance
and 65 would be across terminals 66 and 67 which may
be considered as the “normal” position of the switch.
54 are set to zero and switch contacts 64 and 65 are
element 54 which acted as the reference resistor.
?rst placed across terminals 63 and 69 as shown in FIG
What I claim as my invention is:
URE 3. In this position, a part 76 of ?xed resistor 36
1. A vernier potentiometer comprising: a ?rst series cir~
and all of the tapped resistance 35 are connected in the 60
cuit composed of a source of direct current, a rheostat,
circuit, and by varying the current produced by the direct
current source 31, using rheostat 32, the galvanometer
?rst and second tapped resistances, and a ?xed resistance;
a second series circuit composed of a ?xed proportion of
said ?rst tapped resistance element variably situated there
then returned to “normal” terminals 66 and 67 which
along,
a ?rst current limiting resistor and a variable por
removes the resistance effect of the second tapped resist 65
tion of a third tapped resistance element; a third series
ance 35 and the part 76 of the ?xed resistance 36. While
circuit composed of a ?xed proportion of said second
maintaining the current from the direct current source 31
tapped resistance element variably situated therealong, a
constant, the contacts 41 and 42 are moved from right
second current limiting resistor and a variable portion of
to left along the ?rst tapped resistance element 34 until
the galvanometer 60 indicates that balance has once again 70 a fourth tapped resistance element; a ?rst galvanometer
and a pair of terminals adapted to be connected to a sec
been achieved. The value of the resistance of the ?rst
ond source of direct current connected in series across
tapped resistance element 34 at this point is equivalent to
the combination of said third and fourth tapped resistance
the sum of the second tapped resistance element 35 and
elements; a standard cell and a second galvanometer con
the part ‘76 of the ?xed resistance element 36. In a
vernier potentiometer of this type which was constructed, 75 nected in series across said ?xed resistance, said standard
66 may be balanced.
Switch contacts 64 and 65 are
3,065,418
7
5. A vernier potentiometer according to claim 1 includ
ing switching means comprising a plurality of pairs of ter
minals, each of said pairs of terminals being adapted to
cell and ?rst direct current source having the same polar
ity relative to said ?xed resistance.
2. A vernier potentiometer according to claim 1, in
which said ?rst tapped resistance element is composed of
a series of identical resistances of one value and said sec
be connected to said second source of direct current and
5
said ?rst galvanometer connected in series, a plurality of
reference resistance elements, different combinations of
said tapped resistance elements and reference resistance
ond tapped resistance element is composed of a series of
identical resistances of another value, said identical resist~
ances being separated in each case by equally spaced taps;
elements being connected across the remainder of said
said ?xed proportions of the tapped resistance elements
being composed of a speci?ed number of consecutive 10 pairs of terminals, one of said combinations including one
of said tapped resistance elements wherein a predeter
identical resistances in each case, said speci?ed number
mined
group of taps is to be tested.
being determined by the sliding engagement of a pair of
6. A vernier potentiometer according to claim 5 in
contacts with said taps in each case, said contacts ‘being in
which said plurality of reference resistance elements com
?xed space relation to each other.
3. A vernier potentiometer according to claim 2 in 15 prises ?rst, second and third reference resistors, said ?rst
reference resistor ‘being composed of a section of said ?xed
which said third tapped‘ resistance element is composed
resitance element, the sum of the resistance values of said
of tWo equal resistors with a single slidable contact engag
section of said ?xed resistance element and of said second
ing both of said resistors, one of said identical resistors
being connected in series with said ?rst current limiting re
tapped resistance element being equivalent to the total
sistor to one of the pair of contacts connecting said ?rst 20 resistance of ?ve taps of said ?rst tapped resistance ele
and third resistance being elements, the other of said iden
ment, said second reference resistor equivalent to ten taps
tical resistors connected at one end to ‘the other contact
of said second tapped resistance element, and said third
of that pair.
reference resistor equivalent to ten taps of said third
4. A vernier potentiometer according to claim 2 in
tapped resistance element.
which said fourth tapped resistance element is composed 25
of a single resistor having a sliding contact, said single
resistor being connected in series with said second current
limiting resistor to one of the pair of contacts connecting
said second and fourth resistance elements and the sliding
contact of said single resistor being connected to the other
contact of that pair.
References Cited in the ?le of this patent
Publication: “A Switch-Dial Potentiometer Divider,” by
W. K. Clothier in Journal of Scienti?c Instruments, vol.
33, May 1956 at pages l96—l98.